The survivability and reliability of a silicon cell depends primarily upon the nature and composition of its contacts and the means used to protect the cell from environmental degradation. Survivability of space cells, which are subject to high energy particle radiation damage, may be increased by use of silicon base material which is free of detrimental imperfections and impurities, i.e. high resistivity "P" type zero dislocation density, float-zone silicon. Extended life-time of space cells is achievable if the cells can be subjected to very high temperature short-time radiation damage annealing cycles without the cell contacts, the junction, the antireflection coating, or the cover-glass to cell bond degrading during such cycles.
Conventional cells have contacts consisting of a very thin layer of titanium, covered by a very thin layer of palladium, topped by a comparatively thick layer of silver. Organic adhesives are used to bond protective cover-glasses to the cell front surface.
The conventional cell-glass structure is susceptible to degradations from reactions of the cell contacts and the cell surfaces with environmental contaminants. Such reactions are accelerated by high operating temperatures or any high temperature treatment, such as a radiation damage anneal. The conventional contact is additionally susceptible to high temperature degradation because of the presence of titanium and the presence of undesirable impurities in the contact metals. At high temperatures, these detrimental impurities diffuse into the cell surface and degrade the cell material and the cell junction.
Another source of degradation is the adhesive used to bond the cover-glass to the cell. Presently used adhesives are susceptible to degradation from ultraviolet light and from heat.
High survivability is provided in this invention by complete glass-sealing, without the use of adhesives, and by the use of high temperature silver-cerium contacts; these contacts can be sintered at a temperature of 800.degree. C. on 0.15 .mu.m junction depth cells without damage to the cells. Furthermore, this invention provides a cell having a high purity, high perfection, very thin, single crystal silicon wafer base; optical reflection; and high density base carrier-accumulation. The optical reflection and base carrier-accumulation provide exceptionally high open circuit voltages and currents for extremely thin, membrane cells.
The bottom glass-cover is ground and silvered to provide a mirror which reflects the light transmitted through the very thin cell. Conventionally, reflection has been achieved by making a reflective metal back contact over the entire back of thin cells. The method of this invention avoids the decrease in back contact adherence and the completely metal-covered back surface which fabrication of conventional reflecting back contacts necessitates. This invention provides both reflection and freedom to use narrow back contact lines and to fabricate the back contact so as to provide maximum adherence. Furthermore, this invention provides more effective reflection than does a back contact reflector because the glass surface is both ground and silvered, thereby reflecting diffused light. This effectively increases the path length in the cell of the reflected photons, resulting in increased carrier generation and higher currents than achievable with back contact reflectors.
The glass mirror also reflects solar photons whose energies are too low to generate carriers; these exit through the top cover-glass. A conventional cell mounted on a support substrate will absorb the low energy photons in its back contact and in its substrate. This raises the operating temperature of the cell. This invention provides for removal of the low energy photons so that the cell operates at a lower temperature than do conventional mounted cells. Thus, in this invention, it is not required that the top cover-glass be bonded to the top contact fingers, as described in U.S. Pat. No. 3,541,679, in order to lower the operating temperature. This invention thus provides an important simplification with respect to that described in U.S. Pat. No. 3,541,679.
In some applications, it may be desirable to use large area metallized cover-glasses for mounting and interconnecting an entire solar cell module (numerous cells bonded to a glass superstrate and interconnected by metallization patterns on the superstrate and on a bottom glass substrate). The entire module may be sealed between the top glass superstrate and the bottom glass substrate by the means provided in this invention.
Whether a single glass-sealed cell or an entire glass-sealed module is concerned, the option is available to either make a glass mirror at the back of each cell or simply not to silver the glass, thereby permitting lower energy photons to exit through the bottom glass substrate, reducing the operating temperature considerably. When the mirror option is selected, best results are obtained by applying a tailored-to-the-reflected-light spectrum antireflection-coating to the back surface of the cell. The non-mirror option is preferable for thicker cells where the photons transmitted through the cell bulk are primarily too low in energy to generate carriers and there is, therefore, little cell current to be gained by reflection of the transmitted light back into the cell.
This invention is most advantageous for providing silicon solar cells having a total thickness of less than 50 microns. No solar cells can be made in this thickness range from single crystal wafers by conventional methods. There are extremely important performance advantages provided by cells having thicknesses less than 50 microns. The most important of these is that higher efficiencies are preserved in such thinner cells after high energy particle bombardment than in thicker cells having identical structures, and made from the same silicon high perfection material. Another performance advantage is that higher open-circuit voltages and much lower open-circuit voltage temperature coefficients are obtained for thinner cells made with the structure described in this invention. This advantage is extremely significant for operation under highly concentrated sunlight.
A major factor in the mechanism of power generation of the high resistivity very thin cell having minority carrier barriers at both the front and back of the base region, described in U.S. Pat. No. 4,338,481 (which patent is incorporated herein by reference for all purposes), is the value of the density of photon-generated carriers accumulated within the base region. Since the vast majority of carriers are generated by solar photons absorbed within 25 microns of the illuminated cell front surface, the density of accumulated carriers is vastly increased when the cell thickness is reduced to 25 microns or less. The high minority carrier lifetime, extremely thin base provided in this invention accumulates the photon-generated holes and electrons; this gives rise to voltages at both the front N.sup.+ P junction and the back P.sup.+ P junction, which are series aiding.
Increased values of cell open-circuit voltage are obtained due to the high density base carrier-accumulation provided in the cell of this invention. The density of base (bulk) accumulated carriers can be further increased by increasing solar intensity and by use of a back reflecting mirror. Under concentrated sunlight, a much greater increase in open circuit voltage occurs for the membrane cell than for thicker cells, and much lower open-circuit voltage temperature coefficients are correspondingly realized. Almost ideal high concentration operation is thus provided by this invention.
This invention provides a highly versatile cell package. The versatility is due to the wide choice provided of base material resistivity and thickness, the option of a back glass mirror, the option of either complete glass sealing or a supporting glass superstrate only, the provision of very high temperature contacts, and the metal to metal bonding of glass to cell and top cover-glass to bottom cover-glass. Options to anneal radiation damage and for additional high temperature processing are provided and the mechanisms of power generation provide increased efficiency and reliability for both 1 sun and concentrated sunlight operation.
It is, therefore, an object of this invention to provide a very thin (total thickness as little as about 15 microns), high perfection, single crystal wafer base, silicon solar cell which has high efficiency, high reliability, and high survivability for both 1 sun and concentrated sunlight applications. It is a further objective of this invention to provide a cell completely sealed in glass in a vacuum or in an inert gas environnent, without the use of adhesives. Another objective of this invention is to provide a glass-sealed cell which can be heated to temperatures up to about 800.degree. C. in any atmosphere without suffering significant degradation of performance or reliability.
It is another objective of this invention to provide a thin glass-sealed cell which has high efficiency and increased survivability under conditions of high temperature operation.
Another objective of this invention is to provide a glass-sealed cell which can be heated to temperatures up to 800.degree. C. without significant degradation of performance or reliability in order to anneal radiation damage.
An additional objective of this invention is to provide a glass-sealed silicon solar cell which can be subjected to further very high temperature processing in a reactive gas environrent, e.g. as required to fabricate a tandem amorphous silicon cell on the top glass surface of the sealed-in-glass single crystal silicon membrane cell.
It is another objective of this invention to provide an extremely thin, sealed-in-glass, silicon solar cell whose current is increased by grinding and silvering the glass at the back of the cell, so as to reflect light transmitted through the cell backwards into the cell.
It is an objective of this invention to provide a method for fabricating a cell as thin as about 15 microns having a single crystal wafer base and a rugged top cover-glass support.
It is a further objective of this invention to provide a method for completely sealing a silicon solar cell in a glass enclosure by bonding metallized regions on the cell surfaces to matching metallized regions on a top and a bottom cover-glass and bonding the cover-glasses to each other along metallized peripheral regions.
Finally, it is an objective of this invention to provide a method for fabricating an extremely thin, single crystal wafer base, glass-sealed, silicon solar cell which has much higher resistance to degradation from high temperatures, ultraviolet radiation, high energy particle irradiation, and environmental factors than conventional cells.